1. Field of the Invention
The present invention relates generally to the field of telecommunications. More particularly, the present invention relates to apparatus and methods for in-service performance monitoring of high speed synchronous digital telecommunications signals.
2. State of the Art
The telecommunications network servicing the United States and the rest of the world is presently evolving from analog transmission to digital transmission with ever-increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with better noise immunity.
With the advent of fiber optic communications, various standards governing the transport of data have arisen. One standard is commonly known as xe2x80x9cSONETxe2x80x9d; the xe2x80x9cSynchronous Optical Networkxe2x80x9d. Details regarding the SONET standard are set forth in Bellcore document TR-NWT-000253 which is hereby incorporated by reference herein in its entirety, as well as documents referenced therein. In the U.S., SONET standards are based on a synchronous transport signal (STS) and in Europe, SDH (Synchronous Digital Hierarchy) standards are based on a synchronous transport module (STM). The STS and STM signals can be configured to different bit rates and designated as STS-n and STM-n where n is an indication of a bit rate multiple. For example, an STS-3 signal has a bit rate which is three times that of an STS-1 signal and an STM-4 signal has a bit rate which is four times that of an STM-1 signal. Although and STM-1 signal has the same bit rate as an STS-3 signal, the signals contain different frame structures.
As set forth in the Bellcore documents, and as exemplified in prior art FIG. 1 which shows a SONET STS-1 frame, SONET signals are sent in a frame format which includes section overhead and line overhead which together are taken as transport overhead (TOH) arranged as columns of bytes, and a payload which is also arranged as columns of bytes. The bytes of the payload, in turn, include xe2x80x9cpath layer overheadxe2x80x9d (POH) bytes. As seen in prior art FIG. 2 which details the TOH and POH bytes, among the TOH bytes are bytes B1 and B2 which are eight-bit interleave parity (BIP-8) codes. Similarly, the path overhead includes byte B3 which is a BIP-8 code byte. The bit error rate of a line is derived from the sum of BIP-8 for STS-1s in an STS-N. In the case of STM signals, the bit error rate is derived from the sum of BIP-24 of STM-1s in an STM-N. See ITU-T Recommendations G.707, G.708 and G.709.
As set forth in Bellcore document TR-NWT-000253, a user selected BER threshold ranging from 1xc3x9710xe2x88x923 to 1xc3x9710xe2x88x929 is then used to indicate signal fail (SF) and signal degrade (SD) conditions for the initiation of an automatic protection switching (ASP). (See Section 5.3 of Bellcore TR-NWT-000253). In addition, there is a maximum detection time requirement and an average detection time objective which depend upon the chosen BER level. For example, as set forth in Table 5-2 of Bellcore TR-NWT-000253, the maximum alarm detection time for a BER of 1xc3x9710xe2x88x923 is 10 ms, while the maximum detection time for a BER of 1xc3x9710xe2x88x925 is one second. The average detection time objective for the BER of 1xc3x9710xe2x88x923 is 8 ms, while the average detection time objective for the BER of 1xc3x9710xe2x88x925 is 300/N ms or 8 ms, whichever is greater, where N is the level of the STS signal (e.g., N=1 for an STS-1 signal). The algorithm used to detect alarm conditions should also be tolerant to burst errors of up to 3 ms.
In detecting and generating alarm conditions based on the BIP-8 code violations, a sliding window algorithm (SWA) is suggested in Bellcore TR-NWT-000253. In particular, Mt consecutive blocks of data are examined, where Mt equals the maximum number of blocks observed before reinitialization (for alarm). If there are mt or more blocks (where mt is a desired flag count threshold number) each having m or more parity violations (where m is a parity violation count threshold) in the Mt blocks, an alarm will be issued. If not, the window of the Mt consecutive blocks is advanced (slid) by a single block, and the calculations are repeated for that set of Mt consecutive blocks. Again, if there are mt or more blocks each having m or more parity violations, an alarm will be issued. If not, the window is again slid, and the calculations repeated.
In implementing the sliding window algorithm for a window size of N frames, the BIP-8 code violation must be checked for the current frame and the previous N-1 frames, and the previous N-1 BIP-8 code violations must be stored. The requirement of checking the BER every frame, and storing the previous N-1 BIP-8 code violations places a large strain on hardware and/or software implementing the SWA. In fact, in order to meet the BER threshold of 1xc3x9710xe2x88x923 with the detection time objective of 8 ms, only hardware or VLSI implementation is feasible. In addition, with the sliding window algorithm, the false alarm rate for declaring a signal degrade or failure is unacceptably high.
Previously incorporated co-owned U.S. Pat. No. 5,724,362 discloses methods and apparatus for generating and clearing an excessive bit error rate (EBER) alarm utilizing a reset window algorithm. The BIP-8 bytes (e.g., B2 bytes) of incoming data blocks (each block being B frames long) of an STSn telecommunications signal are monitored in an xe2x80x9cidle statexe2x80x9d for code violation counts (CV). Upon receiving a data block having a code violation count meeting or exceeding a code violation count threshold (CVSET), a counter is initialized in a xe2x80x9ccrossing calculation statexe2x80x9d, and a window comprising a plurality (W) of blocks is monitored. The counter counts the number of incoming blocks in the window having a CV which meets or exceeds CVSET. If in the crossing calculation state, the count meets or exceeds its own threshold (X), an alarm state is entered and an EBER alarm is set. If not, the system returns to the xe2x80x9cidle statexe2x80x9d. Once in the alarm state, every received block is monitored for its code violation count. The first received block with a CV count of CVCLR (code violation clear) or less initializes a xe2x80x9cclearing calculation statexe2x80x9d which sets a clearing-counter CC. The clearing counter CC is used to count the number of incoming blocks in the window having a CV of CVCLR or less. If the CC count meets a third threshold value Y within the time window, the alarm is cleared and the system returns to the idle state. Otherwise, the system reverts to the alarm state. The parameters B, W, X, Y, CVSET, and CVCLR are user configurable for expected BER thresholds between 10xe2x88x923 and 10xe2x88x929. The methods are also applicable to STM-N signals. Although the methods disclosed are generally effective in bursty error conditions, it is not configurable in any particular way for better performance in bursty conditions.
It is therefore an object of the invention to provide a method for conducting excessive bit error rate (EBER) alarm generation and clearing which is implementable in hardware, software, or firmware.
It is another object of the invention to provide an EBER alarm generation and clearing algorithm which meets maximum detection time and detection time objective standards.
It is a further object of the invention to provide an EBER alarm generation and clearing algorithm which has a reduced false alarm rate relative to the sliding window algorithm.
It is an additional object of the invention to provide an EBER alarm generation and clearing algorithm which is configurable for expected BERs and for expected bursty errors.
In accord with these objects which will be discussed in detail below, the algorithm of the present invention utilizes the following user configurable variables: a detection threshold (THDV), a detection time base (TBDV), a recovery threshold (THRV), a recovery time base (TBRV), and a burst mode indicator (BURST). The thresholds THDV and THRV are sixteen bit numbers representing a number of BIP violations. The time bases TBDV and TBRV are sixteen bit numbers representing the number of frames in the sampling windows for determining alarm detection and recovery. The BURST indicator is a one bit value indicating whether the burst mode is enabled or disabled. The method steps of the invention include configuring the variables listed above, and maintaining a frame count (FC) and an error count (EC). If BURST=0, an alarm condition is detected when ECxe2x89xa7THDV and FC less than TBDV, and an alarm recovery is detected when FCxe2x89xa7TBRV and EC less than THRV. If BURST=1, a potential alarm state is detected when ECxe2x89xa7THDV and FC less than TBDV and an actual alarm condition is detected when the error count exceeds the threshold a second consecutive time. Similarly, when BURST=1, the alarm recovery is detected only when the EC stays below the recovery threshold for two consecutive recovery time base windows. According to the presently preferred embodiment, the frame counter is adjustable to count frames once every 125 microseconds or once every 500 microseconds. This allows the sampling windows to be as large as 32.768 seconds each.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.